This invention relates to a vehicle transmission control system, in particular a method for shuttle shifting a powershift transmission having solenoid valve operated transmission control elements, such as brakes and clutches.
Power shift transmissions are known in the art. A control system for a known powershift transmission such as a Funk 8000 Series powershift transmission is described in U.S. Pat. No. 4,989,470, issued Feb. 5, 1991 to Bulgrien. This powershift transmission includes an initial set of three clutches, an intermediate set of three clutches and a final or directional set of three clutches. The clutches are controlled by nine electro-hydraulic 3-way, 2-position direct acting valves and two electro-hydraulic proportional or pressure modulating valves. The pressure modulating valves modulate the pressure supplied to the direct acting valves associated with the three directional clutches. Such powershift transmissions may be controlled to provide a shuttle shift capability which permits the vehicle to slow down and then change direction of operation without requiring the movement of the gearshift lever through each intermediate gear ratio. One such shuttle shifting method is described in the above mentioned patent to Bulgrien, wherein all clutches in an initial set of clutches are released to disconnect the engine from the drive train, at least two clutches in an intermediate set of clutches are engaged to lock up the transmission, at least one clutch in a final set of clutches is engaged to reduce vehicle speed to zero, then after the vehicle speed has been reduced to zero, one clutch in each set is engaged to engage the desired new gear ratio. Thus, this method requires that a minimum of at least three clutches be operated merely to decelerate the vehicle during a shuttle shift. It would be desirable to decelerate the vehicle during a shuttle shift with fewer clutch operations in order to increase system reliability.